The present invention relates to apparatus and methods for preventing the separation of the welded joint between a thermal sleeve and an elbow forming part of a water recirculation system in a nuclear reactor and particularly relates to apparatus and methods for clamping the sleeve and elbow to one another in situ whereby, should the welded joint fail, the sleeve and elbow are prevented from separating.
In a boiling water nuclear reactor, an annular space is defined between the core shroud and the reactor pressure vessel wall. Jet pumps are located in the annular space for recirculating water in the reactor. Typically, a substantial number of jet pumps, for example, on the order of twenty, are installed in this annular space. Each jet pump comprises a riser, a transition piece adjacent the top of the pump, a pair of nozzles, a pair of inlet mixers, a pair of diffusers and riser bracing. The inlet riser is connected to an elbow in the annular space. The opposite end of the elbow is welded to the end of a thermal sleeve which penetrates the reactor pressure vessel wall and supplies water for recirculation through the reactor via a jet pump. The weld between the sleeve and elbow is typically a full penetration butt weld. It will be appreciated that the weld joint lies within the confined space of the annulus and, as such, access to the weld joint is highly restricted. Moreover, the joint is subjected to the reactor environment and is subject to inter-granular stress corrosion cracking.
Over time, cracks may occur in the weld joint between the inner end of the thermal sleeve and the elbow end. In the event that one or more cracks are propagated, the integrity of the welded joint between the elbow and thermal sleeve can be severely compromised. Leakage flow through a cracked welded joint is not a particular concern in light of the fact that water lies about both the inside and outside of the joint. However, the recirculation system may be severely degraded if the thermal sleeve and elbow separate at the joint. Consequently, it is important that a gross separation of the elbow and thermal sleeve should be prevented even if cracks occur in the welded joint causing leakage flows.
In accordance with a preferred embodiment of the present invention, a clamp assembly is provided for precluding separation of the elbow and thermal sleeve at the welded joint therebetween and which clamp assembly is installed in situ. The clamp assembly comprises a pair of clamp heads, each having a semi-cylindrical surface defining a recess. A plurality of pins project inwardly from the semi-cylindrical surface for reception in corresponding holes in the elbow and thermal sleeve. More particularly, first and second sets of pins are axially spaced one from the other on each semi-cylindrical surface. The pins of each set are circumferentially spaced one from the other. Additionally, the pins lie parallel to one another and extend in a direction parallel to a plane passing through the heads, i.e., the pins extend in a plane normal to a plane containing the axis of the thermal sleeve and intersecting joint surfaces of each head defining the ends of the semi-cylindrical recesses. Also, the pins extend progressively greater distances from the semi-cylindrical surface in an inward direction as the pins are spaced from an apex of the head toward opposite ends of the head. Holes are provided at corresponding circumferentially spaced locations about each of the elbow and sleeve ends and have parallel axes for receiving the pins. Once received about the elbow and sleeve with the pins in the holes, the clamp heads are secured to one another, preferably by bolts. In this manner, in the event of a failure of the weld joint, the elbow and sleeve will not separate from one another.
To install the clamp assembly in situ, fixtures mounting electrode discharge machining (EDM) actuators are lowered into the annular space to form the holes in the sleeve and elbow at the desired circumferentially spaced positions. Once the holes are formed, the clamp heads are lowered into the annular space such that the pins are received in the preformed holes in the elbow and sleeve. When received, the clamp heads are secured one to the other, preferably using remotely operated tools to bolt the heads together. Stabilizing pins also extend through one of the clamp heads and terminate in ends received in recesses of the opposite clamp head. The stabilized pins preclude relative sliding movement between the clamp heads, preventing relative rocking motion and prevent crushing the thermal sleeve.
In a preferred embodiment according to the present invention, there is provided a clamp assembly for preventing separation of a thermal sleeve penetrating a nuclear reactor pressure vessel wall and a jet pump riser elbow at a connection between one another, comprising a pair of clamp heads each having a generally semi-cylindrical surface defining a recess opening along one side thereof for disposition about confronting ends of the sleeve and the elbow adjacent the connection therebetween. Each head includes generally axially spaced first and second sets of clamping pins projecting inwardly from the semi-cylindrical recess and fasteners for clamping the heads to one another on diametrically opposite sides of and about the connection between the sleeve and elbow with the pins receivable in holes in respective sleeve and elbow ends for precluding separation of the sleeve and riser elbow should the connection therebetween fail.
In a further preferred embodiment according to the present invention, there is provided in a nuclear reactor having a reactor pressure vessel wall, a core shroud spaced inwardly of the reactor pressure vessel wall defining a generally annular space therebetween, a jet pump in the annular space for circulating water within the core, a thermal sleeve penetrating the reactor pressure vessel wall, an elbow welded at one end to one end of the thermal sleeve and coupled at an opposite end to an inlet riser for flowing water to the jet pump and a clamp assembly for preventing separation of the thermal sleeve and the elbow at the welded joint therebetween. The clamp assembly includes a pair of clamp heads each having a generally semi-cylindrical surface defining a recess opening along one side thereof for disposition about confronting ends of the sleeve and the elbow adjacent the welded joint therebetween. Each head includes generally axially spaced first and second sets of clamping pins projecting inwardly from the semi-cylindrical recess, and fasteners for clamping the heads to one another on diametrically opposite sides of and about the welded connection between the sleeve and elbow ends with the pins receivable in holes in the respective sleeve and elbow ends for precluding separation of the sleeve and riser elbow should the welded joint therebetween fail.
In a still further preferred embodiment according to the present invention, there is provided in a nuclear reactor having a reactor pressure vessel wall, a core shroud spaced inwardly of the reactor pressure vessel wall defining a generally annular space therebetween, a jet pump in the annular space for circulating water within the core, a thermal sleeve penetrating the reactor pressure vessel wall, and an elbow welded at one end to an end of the thermal sleeve and lying within the annular space, the thermal sleeve and the elbow being adapted for supplying water to the jet pump in the annular space, a method of installing a clamp assembly for preventing separation of the thermal sleeve and the elbow, comprising the steps of providing a pair of clamp heads each having a generally semi-cylindrical surface defining a recess opening along one side thereof for disposition about confronting ends of the sleeve and the elbow adjacent the connection therebetween, each head including generally axially spaced first and second sets of clamping pins projecting inwardly from the semi-cylindrical recess, forming in situ and in the annular space holes in the sleeve and the elbow at locations corresponding to the spacing of the pins about each of the clamp heads, applying in situ and in the annular space the heads to the opposite sides of the sleeve and elbow with the first and second sets of pins penetrating the holes in the sleeve and elbow, respectively and securing in situ and in the annular space the clamp heads to one another about the joint between the sleeve and the elbow with the first-and second sets of pins in the holes of the sleeve and the elbow, respectively, to preclude separation of the sleeve and elbow should the joint therebetween fail.